Coating process



March 12, 1957 P. J. CLOUGH ETAL 2,785,082

COATING PROCESS Filed March 22, 1954 FIG.

Power 5 m M d Y E w. E 1w M mwh m (/5 Cu T G M A M O Y B United States COATING PROCESS Application March 22, 1954, Serial No. 417,868

7 Claims. (Cl. 117-22) This invention relates to coating and more particularly to the coating of uniform layers comprising the group Vla metals. This invention has particular utility in connection with the production of coatings of chromium, tungsten and molybdenum, which are useful as decorative coatings, corrosion-resistant coatings, erosionor wearresistant coatings or as conductive films. This application is, in part, a continuation of our copending application Serial No. 181,366, filed August 25, 1950, and now abandoned.

A principal object of the present invention is to provide new and improved processes for coating individual articles or flexible substrates with uniform films comprising tungsten, molybdenum or chromium.

Still another object of the invention is to provide such coating processes which are cheap, simple and extremely rapid.

Still another object of the invention is to provide such coating processes which require a minimum of capital equipment and which can be operated by unskilled personnel.

Still another object of the invention is to provide a process for increasing the cutting speed and wear resistance of a cutting tool, this process adding only a very minor additional cost to the cost of the cutting tool.

Still another object of the present invention is to provide a process of the above type which permits the rapid deposition of relatively thick corrosion-resistant coatings on numerous types of bases.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the process involving the several steps and the relation and the order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

Fig. 1 is a diagrammatic, schematic, sectional view of one preferred form of apparatus for practicing the present invention;

Fig. 2 is a diagrammatic, schematic, sectional view of another form of apparatus for practicing the present invention.

This invention generally relates to the formation, on various surfaces, of coatings of group Vla metals. In the past, surfaces have been coated with these group VIa metals by decomposition of vapors of the carbonylof these metals of the article to be coated. In some of these processes, vacuum techniques were employed and in none of these processes were the results particularly uniform. The coatings were thin and required long, expensive treating steps during their production. Examatent ples of such processes are described in British Patents 589,966 and 589,977.

In the present invention, the coating step is achieved by providing in intimate contact with the surface a layer of dry, powdered carbonyl of said group Via metal. The surface of the article is then heated to a temperature above the decomposition temperature of the carbonyl for a suflicient length of time to form the group Vla metal layer. The surface temperature of the article is maintained above the carbonyl temperature so that a temperature gradient is provided which decreases away from the surface to be coated. When the article to be coated is an electrical conductor, it is conveniently heated by electrical currents therewithin. When it is a non-conductor, the article to be coated may be heated prior to being contacted by the dry, powdered carbonyl or may be heated from a side thereof which is away from the surface to be coated. In any case, the surface temperature of the object to be coated is above the carbonyl temperature and is sufficiently low so that the carbonyl is maintained between about C. and C. Accordingly, the carbonyl remains below its melting point and solid carbonyl remains immediately adjacent the surface to be coated. During the coating operation, the carbonyl is preferably maintained under a coating pressure on the order of 1 atmosphere.

In one preferred method of practicing the invention, the carbonyl is in the form of finely powdered particles having a grain size at least as small as 60 mesh, and the article to be coated is immersed in a relatively large body of this finely powdered carbonyl. If the article is a conductor, it may be heated by induced currents while immersed in the carbonyl. It is a non-conductor, it should be heated prior to being inserted in the carbonyl powder.

With most articles to be coated, the surface temperature is maintained between about 130 C. and 180 C. However, it has been found that higher surface temperatures are occasionally obtained momentarily during the coating process and can be satisfactorily utilized. For example, in coating massive objects of carbon, which are heated by induced high-frequency currents (e. g., l0,000-400,000 cps.) while immersed in a powdered, solid carbonyl, it has been discovered that peak surface temperatures approaching 300 C. have been momentarily employed. In this particular case, the heating was cyclical, i. e., the power was turned on until gas evolution from the carbonyl indicated decomposition and then the power was turned oif. It is believed that the resultant peak surface temperatures (as measured by a thermocouple) were achieved due to the fact that the skin effect of the induction heating provided for most of the high-frequency induced current flow immediately adjacent the surface. Accordingly, most of the heat was generated immediately adjacent the surface. This heat flowed inwardly towards the unheated portion of the carbon body and outwardly through the carbonyl powder. Since the induction heating was cyclical, the surface did not remain at the peak temperature for a long period of time. In fact, it remained at its peak temperature for less than a second.

In another embodiment of the invention, the powdered carbonyl may be applied to the article to be coated in the form of a slurry and dried before the surface is heated to the decomposition temperature of the carbonyl. This embodiment of the invention is less preferred since it is more difiicult to obtain thick oxidation-resistant coatings by this technique.

Referring now to Fig. 1 there is shown one schematic, diagrammatic representation of one preferred embodi ment of the invention, as applied to the coating of a drill, for example. In this Fig. l, 10 represents a container 'by the coated drills was 14. 'life, the coated-drills had on the average a much higher matically indicated at 14, is heated by a high-frequency induction coil 16 connected to a suitable power supply 17. lf desired, a thermocouple 1 d may be utilizedfor indicating, by means of a meter 19, the temperature of the drill during the treatment thereof. The container 10 is preferably verited through a tube 20, containing a quantity of copper oxide 21. Thetube also includesa calcium chloride water-vapor barrier 22, a pressure surge tank 26, and another container 28, the end of tube 20 bengbelowithe level of a water solution of potassium hydroxide 27 held in container 28. V

In'one preferredexample of the process of the present invention, the powdered carbonyl 12 is prepared by powdering chromium carbonyl Cr(CO)s. The crushed grains of carbonyl powder" are preferably of a size at least as small as 60 mesh, this small grain size'being particularly desirable for small drills, taps and the like where there are small grooves to be coated, The powdered carbonyl is then placed in the container 10, A drill 14 to be coated is cleaned, suchas by washing in acetone or by vapor degreasing with 'trichlerocthylene, and is insorted in the powderedcarbonyl, being twisted ash is inserted so as to force the powdered carbonyl into the flutes of the drill It is also desirable-to tap thecontamer so as to insure intimate contact between the powder and all surfaces'of'the drill to becoated. The high-frequency coil 16 is then'energized to heat thedrill to a temperature above the decomposition temperature of the carbonyl. V This temperature is preferably in the range of about 130 C. toabout 180 C., a preferred temperature being approximately 150 C. This temperature is preferably maintained for about 5 to seconds.

One convenient method of operating the apparatus of Fig. 1, without the use of a thermocouple for directly reading the-drill temperature, is to replace the potassium hydroxide solution 27 with water and then energize coil 16 until bubbles appear in the water 27. This indicates that "decomposition of the carbonyl is taking place, the bubbles being carbon dioxide. The coil is deenergized and the drill is allowed to cool slightly for a few seconds. The coil is energized again until another group of bubbles appears 'in water 27.

Again the power is turned oif. This cycle can be repeated for 3 to 6 times to give a very satisfactory coating.

At the end of either of the above heating periods, the coil 16 is deenergized and the drill is removed. The drill is now ready for use, no additional treatment being necessary other than the removal of those few particles of carbonyl which may have adhered thereto.

'Tests made on high-speed drills, which have been t given a chromiumco'ating by the above techniques, have shown a very substantial increase in both cutting'sp'eed andw'ear resistance. A number of uncoated drills were tested by drilling A inch'holes through a inch block of #347 stainless steel, no lubricant being used to expedite the test. .These drills were under a constant load and were 'run with a constant speed. The average number of holes drilled by the uncoated drills was 9. An equal number of similar drills were'coated as above and subjected to the sa'me'test. The average number of holes drilled in addition to their greater cutting speed than did the 'uncoated drills. For example,

theaverage cutting speed of the coated drills, after having or tube 20. The carbon monoxide generated by thecar- "b'o'nyl "decomposition is converted to carbon "dioxide by passing through the copper oxide 21 heated to 320 C. This carbon dioxide is subsequently absorbed in the potassium hydroxide solution 27 in container 28. When an open container 10 is used, it should be employed under a ventilating hood since carbonyl vapors are considered to be quite toxic.

In another example of the'invention, it was desired to form a chromium=coated igraphite hot-pressing mold which would have increased oxidation resistance. A piece of graphite to be used as the mold was cleaned with acetone and packed in chromium carbonyl. The carbonyl was distributed 's'o thafapprtiximately A inch of'finely powdered carbonyl surrounded the graphite. The graphite was slowly heated by induction from a spark-"gap highfrequency (100,000-400,000 cps.) generator to dry the piece of graphite It was'then heated to a surface temperature of about 280 C. to 300 C. and the power was shut off until the decomposition of the carbonyl had nearly ceased. This was indicated by watching the evolution of. gas through a bubbler tube. At this time, the surface temperature had fallen to about C. The .power was then tu'rnedon momentarily to increase the temperature and start the decomposition again. This cyclic heating was carried on for about three minutes, at which time essentially all of the carbonyl had decomposed and a coating of chromium was obtained on the graphite of approximately 0L0005 inch thick. The efiicienc'y of the decomposition was close to 100% and the weight of chromium deposited was very near the theoretical weight one would calculate. The sample was found to be completely covered with a thin, quite uniform coating and,

in subsequent tests in an oxidizing atmosphere at elevated temperatures, was found to resist oxidation very well. Another sample of carbon, to be used at very elevated temperatures, was coated for 10 minutes by heating cyclically as above while surrounded by a thick layer of chromium carbonyl powder. This procedure gave a chromium'coating about ,5 inch thick. This coated sample withstood an oxidizing atmosphere for over 40 minutes while heated to 1550 C. An uncoated carbon samplesubjected to the same test was burned through in less than 5 seconds.

Inmany cases it is highly desirable to coat continuous substrates with metals, and the present invention is particularlyadaptable for accomplishing such coatings. Such coatings can be applied by techniques, to be described hereinafter, to a large group 'ofmaterials such as metal sheets, plastic substrates, fabrics and the like. In Fig. 2 there is shown an embodiment of the invention which employs the above described processes for applying metallic coats to long strips of flexible substrate. Inthis embodiment of the invention, the substrate is moved past a station for applying the group VIa metal carbonyl powder ther'eto,.this powder being applied in the form of and 76 support the substrate during its passage through the coating chamber, platen 76 being arranged 'sothat 'it "'c'anbe heated to a-t'emper'ature above the decomposition temperature of the carbonyl.

Suitable advancing and guide rolls 78 are provided in various locations within the 'coatingchamber for assisting in moving the substrate 72 through "the coating chamber 70. A supply'of the ca'rbonyl powd'er 80 is shown as being held in a hopper =81 so that the powder may be spread by means of a doctor blade 82 in a thin layer ontothe surface of the substrate 72as-this substrate moves past the. doctor blade '82. -A brush 84 is provided-for removing-any carbonyl powder remainin on the substrate 72=-after the coating operation, this excess powder falling into a bin 86 which may, if desired, be returned to hopper 81. If desired, an additional heating means, illustrated as heat lamps 88, may be provided for preheating that surface of the substrate 72 which is to be coated by decomposition of the carbonyl. In a preferred form of the invention, there is provided a vapor shield 90 between the heat lamps 88 and the remainder of the interior of the coating chamber. The coating chamber 70 preferably also includes an exhaust system 92 in front of which there is positioned a cooling coil 94 which is arranged to condense any carbonyl vapors which might otherwise be withdrawn by the exhaust system 92.

In the use of the Fig. 2 embodiment of the invention, substrate 72 is positioned in the coating chamber and a supply of carbonyl powder is placed in the hopper 81. Platen 76 is then heated, for example by steam, to a temperature on the order of 150 C. If desired, heat lamps 88 may also be energized to preheat the substrate as it moves through the apparatus. In a preferred embodiment of the invention, the preheating of the substrate is limited to a temperature of less than about 120 C.

As soon as the above heating means are placed in operation, the substrate is advanced through the coating device by means of rolls 78. During this advancement, the doctor blade applies a layer of powdered carbonyl 80 to the surface of the substrate, this powder layer being approximately 5 inch thick and being substantially uniformly spread over the surface of the substrate to be coated. As the powder-coated substrate passes across the highly heated platen 76, the substrate becomes heated to about 150 C., thereby decomposing at least that portion of the powdered carbonyl which is in intimate contact with the surface of the substrate, thus forming the metal coat thereon. As the coated substrate passes the brush 84, any remaining carbonyl powder is brushed off. Any undecomposed carbonyl vapors generated by heating of the carbonyl above its decomposition temperature are trapped by cooling coils 94 so that they may be recovered for subsequent use. By means of the above apparatus and method, numerous substrates may be coated with various metals by the decomposition of the group VIa metal carbonyls.

When using carbonyls of tungsten and molybdenum in place of chromium carbonyl, the abrasion resistance and cutting speeds of cutting tools treated thereby are increased, but not by as great an amount as occurs when chromium is employed. However, these coatings are particularly advantageous for high-temperature-resistant materials. In using molybdenum carbonyl, the temperature of the surface of the article to be coated should be on the order of about 160 C., while for tungsten carbonyl, the temperature is preferably on the order of about 170 C. As explained previously, these temperatures are not critical, but it is preferred to operate at temperatures such that the carbonyl powder is not much in excess of 180 C. At article-surface temperatures above 180 C., the carbonyl powder adjacent the article surface may be vaporized very rapidly, tending to waste the relatively expensive carbonyl. However, as pointed out previously, particularly in connection with the coating of the graphite die, these high article-surface temperatures are not disadvantageous in those cases where the heating is intermittent and the surface is only momentarily at the peak temperature. In fact, this cyclical method of heating is preferred since it enables operation at maximum temperatures so as to provide the highest possible coating rate. With such cyclical heating it is readily possible to obtain coatings of chromium on carbon as thick as A inch in 10 minutes. The thicker coatings achieved at the higher temperatures generally appear to give poorer results for abrasion-resistant coating. However, corrosion-resistant and erosion-resistant coatings obtained at these higher temperatures with their resultant greater thickness are usually more advantageous.

In the preferred processes described above, the article to be coated has been shown as being heated while in contact with the powdered carbonyl. While this is a preferred system of operation, it is not essential to the process. For example, the article to be coated may be heated to or slightly above the desired operating temperature prior to its insertion into the powder and can then be inserted in the powder before any substantial cooling takes place. This method of operation is particularly suitable where the article to be coated has a relatively large volume of considerable heat storage capacity so that the temperature drop during the coating process is not unduly great. Equally, the carbonyl powder may be applied to the article in the form of a slurry of the carbonyl powder in water or an organic vehicle such as acetone. In this case, the slurry is dried and the article is then heated to the preferred decomposition temperature. With this method of operation, it is still preferred that the article be so heated '(e. g.', by induction) that the surface to be coated is at a higher temperature than the carbonyl powder adjacent the surface. In the Fig. 2 type of coating employing a slurry, the apparatus would involve a separate drying step between the application of the slurry and the decomposition of the carbonyl.

The expression group VIa metals" is intended to in-' clude those metals in group VIa shown on the Periodic Chart of the Atoms, W. F. Meggers, 1947 edition, published by W. M. Welch Manufacturing Company.

While the articles coated by the processes described above were either metal (e. g., drill) or carbon (e. g., carbon filament), numerous other materials can be coated. About the only limitation which is placed on this coating process is the heat sensitivity of the substrate to be coated. The substrate must be able to withstand temperatures on the order of C. to C. without decomposition, softening or outgassing. Accordingly, numerous materials such as metals, plastics, glass, ceramics, etc. can be provided with coatings of the group VIa metals by the present invention. As mentioned previously, these coatings may be for the purpose of increasing the resistance of the coated article to corrosion, erosion or abrasion. Equally, the coating may be for the purpose of increasing the hardness or electrical conductivity of the article surface or it may be solely for decorative purposes.

Since certain changes may be made in the above products and process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. The process of coating the surface of an article with an adherent coating of a group VIa metal, said process comprising the steps of providing in intimate contact with said surface a layer of dry powdered carbonyl of said group Vla metal while said surface of said article is maintained at a temperature above the decomposition temperature of said carbonyl for a sufiicient length of time to form said layer, the surface temperature being above the carbonyl temperature and being sufficiently low so that the carbonyl is maintained between about 130 C. and 180 C. so as to be below its melting point and solid carbonyl remains immediately adjacent the surface, said surface remaining below the temperature at which said surface can react with said carbonyl to form a volatile compound, and maintaining said carbonyl under a total pressure of about one atmosphere during the decomposition of said carbonyl, the article being maintained at a temperature which is higher than the temperature of the surrounding solid carbonyl powder during the decomposition of the carbonyl.

2. The process of claim 1 wherein said carbonyl is applied to said surface in the form of a slurry and then dried before the surface is heated to the decomposition temperature of the carbonyl.

3. The process of 'coating thesurface of arr article with an adherent-coating of a-group-VIZtrnet-al, said process comprising'the-steps' of providing intimate contact withv said surface-a layer-of -dry, powdered carbonyl of said group VIa metal,heating'the a-rticle sothat the surface thereof-is maintained at-a temperature above thedecomposition temperature of said carbonyl for a su'fiicient length of time to form-said 1ayer,'thesurface temperature beingabove the carbonyl temperature and 'being'sufiiciently low'so that the carbonylis maintained between about 130 C. and 180 C. so as to be'below'its melting point and solid carbonyl remains immediately adjacent the surface, said surface remaining-belowthetemperature at which-saidsurfacecarrreact with said carbonyl to form a volatile compound, andmaintaining-said"carbonyl under a total pressure ofabout oneatmosphere during the decomposition of said carbonyl, the article being maintained at a temperaturewhich ishigherthanth'etemperatureof the-surrounding solid carbenyl'powder during the decomposition-ofthe-carbonyl. W I

4. The process of claim 3 wherein thearticle'is-heated cyclically by induced high-frequency electrical currents, the temperature of-the article surface risingmomentarilyabove 180 C. during'the-heating cycle-.-

' 5. The process of claim- 1 wherein-themateria1 tobe coated-is heated to the decomposition temperature prior to contacting the'carbonylpowder; V

6; The-process of claim 1 wherein the article to be coated is a conductor of electricity and "is heated to'the decomposition temperature by electrical losses due to the electrical resistance-- of the' article-and" electrical currents j flowing therein.

7. Theprocess of coating the surface of {anarticlewith an adherent coating of agroup: VIa metal, ,saidprocess comprising'the steps of packingsaid article ina mass of 'finely powderedearbonyl of said group'VIa metal,

saidcarbonyl-having a-grain size-atle'ast'as small as mesh, then-heating saidarticle surface'toa temperature above the decomposition temperature; of said carbonyl for a suiiicientl'ength of time 'toform said layer, thesurface temperature being above the carbonyltdecomposition.

temperature and'being sufficiently lOWSO that the carbonyl is maintained between aboutl30 C. and C. so as tobe below itsmelting point and solid'carbonyl remains adjacent the'surface; ,said surface remainingbelow" the temperature atwhich said surface can react With'said; carbonyl to'form a volatile" compound, 'andmaintainingj said carbonylunder a'total pressure of about one atmosphere duringthe decornpositi'on of'said carbonyl, the artiole being maintained at aternperature which is' higher than the-temperature of the surroundingsolid' carbonyl;

powder-during the decomposition of the'carbonyl; 

1. THE PROCESS OF COATING THE SURFACE OF AN ARTICLE WITH AN ADHERENT COATING OF A GROUP VIA METAL, SAID PROCESS COMPRISING THE STEPS OF PROVIDING IN INTIMATE CONTACT WITH SAID SURFACE A LAYER OF DRY POWDERED CARCONYL OF SAID GROUP VIA METAL WHILE SAID SURFACE OF SAID ARTICLE IS MAINTAINED AT A TEMPERATURE ABOVE THE DECOMPOSITION TEMPERATURE OF SAID CARBONYL FOR A SUFFICIENT LENGTH OF TIME TO FORM SAID LAYER, THE SURFACE TEMPERATURE BEING ABOVE THE CARBONYL TEMPERATURE AND BEING SUFFICIENTLY LOW SO THAT THE CARBONYL IS MAINTAINED BETWEEN ABOUT 130*C. AND 180*C. SO AS TO BE BELOW ITS MELTING POINT AND SOLID 